1. Field of the Invention
The present invention relates generally to high mechanical resistance Al—Mg type alloys, and more particularly, to alloys intended for welded constructions, such as those used for motor car body panels and constructions, industrial vehicles and fixed, mobile tanks and the like.
2. Description of Related Art
To increase the mechanical resistance of welded constructions while decreasing their weight, it is of interest to have, with respect to the 5083, 5086, 5182, 5186 or 5383 alloys currently used, enhanced mechanical characteristics without losing any of the properties generally desirable for end use applications. These properties include weldability, corrosion resistance or formability, particularly in low cold-worked tempers such as the O temper and the H111 temper. (The designation of these alloys follows the rules of The Aluminum Association and that of the metallurgical tempers is defined in the European standard EN 515.)
To design a structure, the parameters governing a user's choice are essentially the static mechanical characteristics, that is, ultimate tensile strength Rm, tensile yield strength Rp0.2 and the elongation at fracture A. Other parameters which are involved, according to the specific requirements of the target application, include the mechanical characteristics of the welded seam, the corrosion resistance of the sheet and the welded seam, the fatigue strength of the sheet and the welded seam, the crack propagation rate, the fracture toughness, the bendability, the weldability, the propensity for residual stress formation under specific sheet manufacturing and usage conditions, as well as the ability to produce sheets of uniform quality with the lowest possible production cost.
The state of the art offers several processes to enhance the mechanical characteristics to Al—Mg type alloys. For example, EP 769 564 A1 (Pechiney Rhenalu) discloses an alloy of the following composition (percentage by weight): Mg 4.2–4.8 Mn<0.5 Zn<0.4 Fe<0.45 Si<0.30 where Mn+Zn<0.7 and Fe>0.5 Mn. The alloy may also contain other elements, making it possible to manufacture sheets having an Rm>275 MPa, A>17.5% and an Rm×A product>6500 in a low cold worked state. In a better-controlled composition, it is possible to increase the Rm×A product to a value greater than 7000 and even greater than 7500. Alloys of this type are used under the Aluminum Association reference 5186 in welded road tanker construction. For this application, the Rm×A product is used as a parameter to estimate the behaviour of the structures under deep plastic deformation, for example in the event of an accident. Those skilled in the art know how to increase, in any of the known Al—Mg type alloys, one of the two parameters Rm and A to the detriment of the other. EP 769 564 A1 discloses that sheets with an improved compromise between said two parameters may be obtained if the sheet has a very particular microstructure. The 5186 alloy sheets are characterised not only by a high Rm×A product, but also by a high value of A, which favours the bending of the sheets and facilitates their use in mechanical construction.
Another process is proposed by the patent application JP 62 207850 (Sky) which discloses alloys of the following composition (percentage by weight):
Mg 2–6Mn 0.05–1.0Cr 0.03–0.3Zr 0.03–0.3V 0.03–0.3and which may also contain Cu 0.05–2.0 and/or Zn 0.1–2.0. These alloys are produced by continuous casting and the intermetallic particle size thereof is less than or equal to 5 μm. Such alloys would be useful for manufacture of sheets for motor car bodyworks, since such alloys could be subjected to very particular thermo-mechanical treatment procedures, in order to form sheets of a thickness of 1 mm, which in turn, do not show Lüders lines.
Another process is proposed by EP 0 892 858 B1 (Hoogovens Aluminium Walzprodukte GmbH) which discloses alloys of the composition
Mg 5–6Mn 0.6–1.2Zn 0.4–1.5Zr 0.05–0.25and which may also contain other elements. Such an alloy could potentially be used to manufacture very hard alloys, particularly with a zinc content of the order of 0.8%. These products show an elongation at fracture not exceeding a value of the order of 10% in the H321 temper and 20% in the O temper.
EP 823 489 B1 (Pechiney Rhenalu) discloses products of the following composition
3.0 < Mg < 6.50.2 < Mn < 1.0Fe < 0.80.05 < Si < 0.6Zn < 1.3and which may also contain other elements. These products are characterised by a very particular microstructure, and are not devised to be used for tanker construction, but for welded constructions used in contact with seawater or in a maritime environment.